“Implementation of a Cooperative MAC protocol using a Software Defined Radio Platform”
Center for Advanced Technology in TelecommunicationsCenter for Advanced Technology in TelecommunicationsPolytechnic Institute of NYUPolytechnic Institute of NYU
Brooklyn, NY 11201Brooklyn, NY 11201
Thanasis Korakis
Joint work with Ankit Sharma, Vikas Gelara, Shashi Raj Singh,Pei Liu, Shivendra Panwar
Outline
MotivationCooperative MAC (CoopMAC) ProtocolImplementation effortsExperiment resultsConclusions – Future Work
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Motivation for Cooperative Communications
Use other wireless nodes To relay informationFor robustness to channel losses and variations
Results inSpatial diversity, higher data rates, power saving, extended coverage, higher aggregate network performance.
Motivation for Cooperative MAC protocols
Wireless channel by nature is a broadcast one.The broadcast channel can be fully exploited for broadcast traffic.But it is considered more as a foe than a friend, when it comes to unicast.
STA1
STA3
STA2
IEEE 802.11b: 1Mbps, 2Mbps, 5.5Mbps and 11Mbps.
Primary objective: Combat adversary wireless channel conditions and deliver packets with acceptable BER/PER.
Basic principle: Adjust the modulation scheme and transmission rate, based upon the perceived channel condition
-5 0 5 10
10-4
10-3
10-2
10-1
100BER vs. SNR
BER
11Mbps5.5Mbps2Mbps1Mbps
SNRSNR
BER
11Mbps
1Mbps
Motivation for Cooperative MAC protocols
Multi-rate Capability of Wireless Protocols
Turn a Foe into a Friend?
Inefficiency in 802.11 when different rates are used from different stations“Slow” stations lower their throughput as well as the aggregate throughput of the network.
A cooperative MAC protocol would leverage both the broadcasting and multirate capabilities of the existing MAC protocol
STA1STA2
WithoutCooperation
T1 (STA1 STA3)
11Mbps
11Mbps
T2 (STA2 STA3)
WithCooperation
T3 (STA1 STA2) T4 (STA2 STA3) T2 (STA2 STA3)
1Mbps
11Mbps11Mbps
STA3
Throughput gain
Outline
MotivationsCooperative MAC (CoopMAC) ProtocolImplementation effortsExperiment resultsConclusions – Future Work
STA1 STA2
Access point
Protocol Definition
Procedure for selecting a helperEach station maintains a table (CoopTable) with information about neighboring stations and their ability to helpBased on CoopTable, find a helper that satisfies the following condition
Time[Direct Tx] > Time[Two-hop Relaying]
Data transmission procedureHandshake
RTS (Ready To Send)HTS (Helper ready To Send)CTS (Clear To Send)
Data transmissionAcknowledgement
RTSHTSCTS
DataACK
Outline
MotivationsCooperative MAC (CoopMAC) ProtocolImplementation effortsExperiment resultsConclusions – Future Work
Implementation approaches
PlatformBased upon HostAP driver on a Linux platform.Modified driver for CoopMACimplementation.
Limitations Faced: Inaccessibility of the firmware for modification.
Time sensitive functions (e.g. transmission of a Packet (ACK etc.) in SIFS interval) are controlled by the firmware. RTS-CTS functionality is controlled by the firmware.
IEEE 802.11b PHY
IEEE 802.11 DCF MAC
Intersil Prism chipset
Firmware on the chip
HostAP Driver in Linux kernel
Protocol stackProtocol stackPrevious Work
This implementation missed several critical functional characteristics of the original protocol due to above limitations.
Drivers Implementation
Challenge 1To replace the initial control packets (RTS-CTS) with the RTS-HTS-CTS packetsSIFS (e.g., 10ms for IEEE 802.11b) time as the interval between two consecutive control messages.
Approach: We suspend the initial control frames, as we can’t control the timing of the transmission of those frames.
802.11802.11 CoopMACCoopMAC
RTS
HTS
HCSS
D
S H
D
CTS
CTS
RTS
RTS
CTS
Challenge 2: To transmit the data packet from the helper to receiver without competing for the medium (SIFS period after its reception).Approach: As we can’t control time sensitive functions of MAC (they are controlled by the firmware), we go with the approach of medium contention during the second hop transmission, which increases the overhead.
Data
Data
S H
D
Drivers Implementation
Challenge 3: to suppress the ACK at the helperApproach: Keep the ACK from the destination, at the expense of living with the ACK from the helper.
CoopMACCoopMAC Our approachOur approach
Data
Data
Data
Data
ACK
ACK
S H
D
S H
D
Drivers Implementation
ACK
CoopMAC implementation
In order to overcome those limitations, we had to go from the drivers implementation to an all software radio implementation:Two options:
WARPGNU Radio
We chose WARP:More powerfulRealistic transmission rates (up to 54Mbps)Convenient to build cooperation in the PHY layer
WARP PlatformSoftware defined radio platform developed by Rice University.Xilinx Virtex II Pro FPGA board with embedded power PC Processors.OFDM based PHY
Loosely Based on 802.11a standard.2.4GHz/5 ISM/UNII bands for transmission.
Provides a MAC framework called WARPMAC.Both the PHY and the MAC layers can be changed to any extent.
SDR ImplementationTwo operational modes for transmission
Direct mode: legacy direct mode under the CSMA protocol.Cooperative mode: that enables CoopMAC. Packet is forwarded to the destination through the helper using two fast hops.
Enhanced packet structure called CoopFrameCoopDestinationID: new subfield in header. Used in cooperative mode and indicates the final destination of the packet.Two new packet types (PktType subfield):
COOPPACKET: used in CoopMAC for the first hop transmission (source to helper).COOPFINAL: A packet that is used in CoopMAC for the second hop transmission
Functionality modificationsTransmitter
Based on the CoopMAC table, selects the cooperative mode or Direct mode.Based on the mode, the header of Coopframe is constructed.
Receiver Based on the type of the packet it receives:
DATAPACKET: an ACK is transmitted back to the source node.COOPPACKET: the receiver realizes that it should react as a helper. It replaces the Destination Address field with that of the final destination address based on the CoopDestinationID field, and forwards the packet immediately (defining it now as a COOPFINAL packet), without contending for the channel.COOPFINAL: sends back an ACK, directlyto the source node.
CoopMAC Table
Under the CoopMAC protocol, a station updates its CoopMAC Table passively, based upon the overheard packets exchanged between its neighbors.
In addition to the passive mechanism, an active approach has also been taken in the implementation.
Every station broadcasts a Hello packet, which explicitly indicates the rate between itself and the neighbors.
This is used to create a more controllable experiment environment.
Every time a station receives a Hello Packet, it updates the corresponding record in the CoopMAC Table or it adds a new entry.
CoopMAC Table
Outline
MotivationsCooperative MAC (CoopMAC) ProtocolImplementation effortsExperiment resultsConclusions – Future Work
Experimental StudyScenario 1
Shows the gains obtained by boosting the transmission rates for fixed PER.
Scenario 2Shows the gains obtained by decreasing the PER while fixing the transmission rates.
Basic experimental settingsa. Iperf is used to generate UDP traffic.b. Packet size:1470 bytes
6Mbps
24Mbps24Mbps
high
PER
Performance Evaluation
Experiment settings: Direct transmission: 6Mbps.Cooperative transmission: both hops 24Mbps.
Experimental settings: 1470 bytes payload size
Scenario 1
Major findings: CoopMAC (with or without contention) performs better than the direct transmission.New implementation (CoopMAC without contention) performs much better than our earlier implementation (CoopMAC with contention).
Performance Evaluation
Experiment settings: Transmission rate for the direct & the two hops of cooperative is 6Mbps.Throughput and PER for traffic load of 1Mbps.
Major findings: Throughput of CoopMAC is almost double than that this of direct transmission. PER for the direct transmission is very high (higher than 40%).Cooperative scheme keeps the PER of the communication at a very low level (less than 2%) and therefore increases the efficiency of the network.
Scenario 2
This work explored the full potential of a Cooperative MAC protocol by implementing it on a software defined radio platform.
Open source drivers limitations overcameFeasible to be implementedMinimal changes to the 802.11 protocolSignificant performance boost
Future workImplementation of cooperative schemes in the PHY layer using the WARP platform, and combine them with the existing cooperative MAC protocol.
Conclusions – Future work
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Thank you!
Check the project’s site: http://witestlab.poly.edu
